Conversion coatings are widely used with metals, including aluminum and aluminum alloys and magnesium and magnesium alloys, among others, in which a treatment solution is applied to the metal surface and the metal surface reacts with the treatment solution to convert to a corrosion protective film. In many instances, the protective film serves as a primer which may be top-coated with a paint for aesthetical purposes and to provide enhanced corrosion resistance.
Historically, conversion coatings have employed chromates where maximum corrosion protection is desired or required. One of the most widely used chromate treatments for aluminum and aluminum alloy substrates is a chromate-containing process which produces a protective coating on aluminum that ranges in color from light iridescent gold to tan. This process can be operated at room temperature and the coating produced minimizes corrosion and provides an improved bond for subsequently applied paint. This process is based on soluble chromates containing CrO4− as an inhibitor species and provides a robust conversion coating on various aluminum and aluminum alloy surfaces. For example, chromate conversion coating on Al 2024 T-3 aluminum alloy panels has been demonstrated to resist salt spray exposure in excess of 300 hours without pitting.
However, chromate processes produce chromate waste and residue which must be disposed of and which are either not permitted or are severely restricted by the U.S. Environmental Protection Agency (EPA) and other similar environment, health and safety regulators. Due to heightened concerns over the negative health and ecological effects of hexavalent chromium and chromium salts, there continues to be an ongoing effort to develop acceptable alternatives to chromate conversion coatings.
While various efforts have been made to avoid the use of chromates in conversion coatings, special treatments are often required which are either unacceptable to the user or that do not provide the required level of corrosion protection. In addition, while products have been developed that have the same or similar performance to hexavalent chromium conversion coatings, these products typically do not provide any color to aid in distinguishing a treated surface from an untreated surface or produce inconsistent color or discoloration that make the appearance of the treated surface undesirable to the user.
U.S. Pat. No. 6,412,642 to Charles et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method of applying a marking agent to metal stock, such as aluminum sheet, for scrap sorting purposes in order to provide a detectable mark on pieces of manufacturing scrap so that scrap can be sorted and the marked alloy scrap pieces separated from scrap of other alloy sheet to which the marking agent is not applied. In this instance, the marking agent is added to a lubricant or prelubricant that is typically applied to the sheet by the sheet producer prior to the manufacturing customer. Examples of the marking agents include visible dyes, pigments, fluorescent dyes, and compounds having distinctive infrared absorption spectra However, Charles does not recognize the use of marking agent in prepaint conversion coating compositions or the particular problems that may arise if one attempts to incorporate marking agents into the prepaint conversion coating composition.
In particular, the addition of organic dyes to conversion coating compositions typically leads to higher coating costs, and more importantly, complicates the chemistry of the conversion coating solution which leads to difficulties in controlling the process solution. Thus, it would be desirable to develop a conversion coating composition for aluminum and aluminum alloy substrates that incorporates a suitable marking agent and that provides good corrosion protection while at the same time maintaining good bath stability.
U.S. Pat. No. 6,602,582 to Winterowd, the subject matter of which is herein incorporated by reference in its entirety, describes a composition for treating the edge of a wood-based panel. The composition is a colorless edge sealant that includes an optical brightener which acts as a latent visual marker that can be observed by exposure to UV light. However, this composition is limited to use of treating edges of wood-based panels and again there is no suggestion that such a visual marker would be usable in conversion coating compositions.